What are the benefits of intermittent fasting? Experts' verdict on the 16:8 plan

New studies suggests intermittent fasting can give the body time to heal and renewal

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The 16:8 diet plan involves intermittent fasting, and celebrities like Jennifer Aniston and Hugh Jackman have reportedly given it a thumbs up.

Similar to the 5:2 diet plan - but not as restrictive - the 16:8 is one of the most popular celebrity diets. Apparently, Jennifer Aniston fasts for up to 16 hours to maintain her figure - claiming she's 'not a breakfast person' - while Hugh Jackman's Wolverine physique was fuelled by 16:8 fasting.  

Clinical nutritionist Suzie Sawyer says this style of eating is very different to most of the other diets out there, and that's why it works. "Intermittent fasting works by increasing the period when your body has burned through the calories eaten during your last meal and begins burning fat," she explains. "If you are someone who eats three meals a day, plus snacks, and you don't exercise much, then every time you eat, you are running on those calories and not burning your fat stores." 

But how does the 16:8 diet work, and is it safe? We’ve spoken to the experts to find out all you need to know if you're considering trying the 16:8 diet.

What is the 16:8 diet plan?

The 16:8 diet is a form of intermittent fasting where you break the day into two parts: a 16-hour fasting period and an eight-hour period where you consume all your calories - hence why it is also known as the 'eight-hour diet'.

The 16:8 stems from the 8-Hour Diet book by author David Zinczenko and Peter Moore, former editor-in-chief of Men's Health. The two created the plan off the back of the popular 5:2 diet, with the main difference being the longer fasting time between eating sessions. This, they say, gives the body the time it needs to process food and burn away extra fat.

Suzie Sawyer, founder of Nutrition Lifestyle, explains, "There are no restrictions on the types or amounts of food that a person can eat during the 8-hour window. This flexibility makes the plan relatively easy to follow." 

How does the 16:8 diet work?

The 16:8 diet works on an hourly basis, so each day you eat within an 8-hour time frame and then fast for the remaining 16 hours. This works for weight loss as it "helps you eat fewer calories while boosting metabolism slightly," says Sawyer.

She adds, "It's a very effective tool to lose weight and visceral fat. After hours without food, the body exhausts its sugar stores and starts burning fat - sometimes referred to as 'metabolic switching'."

Research in the Nature Review Neuroscience journal has shown this switch is a natural process that happens when the body runs out of energy from food. The body changes its energy source from glucose that's stored in the liver to ketones, which are stored in fat cells. While the body is getting its energy from glucose it's maintaining or gaining weight, but when the switch happens weight is lost.

However, unlike calorie-restricted diets, short-term intermittent fasting helps to speed up your metabolism - by as much as 14%, according to research from Johns Hopkins Weight Management Center. It does this by causing a rise in norepinephrine, a stress hormone responsible for releasing fatty acids from fat cells and making larger amounts of fat available for the body to burn. The faster your metabolic rate, the more calories you'll burn by doing basic activities like sitting down, eating and sleeping. Exercise is also encouraged during intermittent fasting, which in turn helps with hanging onto and building muscle mass - another essential way to boost your metabolism.

However, while fasted workouts have their benefits, it’s important to keep in mind that low-intensity and short-duration workouts are better options than longer or high-intensity workouts. If you are doing a high-intensity workout, this should be done outside of the fasting window, and not too long after you've had a meal. Make sure to stay hydrated, and take a break if you begin to feel dizzy or lightheaded. 

When can you eat on the 16:8 diet?

If you're following the 16:8 diet, you can pick any 8-hour window to suit your day. 

"The most common hours adopted for the eating period is 12pm to 8pm," explains Tom Jenane, nutrition and fitness expert at Nature's Health Box. "The reason for this is because people aren't normally that hungry in the morning and you don't want to be consuming too many calories in the evening. This window allows for lunch and dinner as well as healthy snacks."

But while the choice is yours, research has identified the best time to eat breakfast, lunch and dinner for weight loss. According to a study by the University of Murcia, it's best to stick to a breakfast in the morning, as skipping the meal is linked to a higher rate of obesity. Meanwhile, a late lunch (after 3pm) can hinder weight loss in those trying to reduce fat mass, and a late dinner (within two hours before going to sleep) decreases glucose tolerance, making it easier to gain weight.

What can you eat on the 16:8 diet?

  • Whole grains: Rice, oats, barley, wholegrain pasta and quinoa.
  • Protein: Meat, poultry, fish, eggs, nuts and seeds.
  • Fruit: Apples, bananas, berries, oranges and pears.
  • Vegetables: Broccoli, spinach and other leafy greens.
  • Healthy fats: Olive oil, coconut oil, avocados.

The great thing about the 16:8 diet is that you can eat anything you want, but it's important to choose whole foods and those high in vitamins during the eating period. 

"Many people fail the 16:8 diet by packing in too many calories into the 8-hour period, often trying to get some in before the 8 hours ends,” says Tom Jenane. “You should still be following a strict diet with a complete nutritional breakdown, to ensure you are consuming a targeted number of calories, not to mention macronutrients and ensuring you’re not consuming too much sugar.”

Meanwhile, it's important to remember that the NHS recommends that the number of calories that should be consumed per day is 2,500 calories for a man and 2,000 for a woman. You need to make sure your body is getting enough fuel outside of the fasting period. 

How much weight can you lose on the 16:8 diet?

If done so correctly, there's a typical weight loss of around seven to 11 pounds over a ten-week period, according to a review of studies by Sydney Medical School. They found, on average, someone weighing 90kg would lose 5% of their body weight in 10 weeks.

But to lose weight on the 16:8 diet you must be eating fewer calories than you're burning - known as a calorie deficit. According to a leading study by the University of Vienna, this energy deficit can be achieved in many ways - but is ultimately the only method of weight loss. 

This is because when you eat food, your body metabolises it to create energy used for daily activities - everything from breathing to walking around. If you have more energy than your body uses, the extra is mainly stored as fat. If you have less energy than what your body can burn, your body will turn to fat stores for energy. This is the calorie deficit.

While intermittent fasting has benefits, eating in an eight-hour period alone will not automatically mean you lose weight. The plan is just one way to achieve the deficit without having to count how many calories you burn in a day. The given eating time aims to automatically restrict the amount of food you can eat, leading to weight loss.

How often should you do 16:8 intermittent fasting?

You can do anywhere from one day of intermittent 16:8 fasting to seven days a week, because, unlike other intermittent fasting diets, each day of the 16:8 works independently to the rest. 

Evidence differs, however, on whether it's healthy to do intermittent fasting all the time. While research from Queen's Medical Centre has proven that short-term fasting boosts the metabolism, longer fasts of around three or more days can actually suppress it and slow it right down. Plus, going without food for a long period of time can lead to an increased level of cholesterol, feeling sick in the morning, dehydration and spells of low blood sugar.

Meanwhile, healthcare company Bupa explains that there isn't enough evidence to show what would happen if you tried this type of fasting over a long period of time.

There are also a number of reasons why intermittent fasting may not be suitable for you:

  • if you are pregnant
  • have type 1 diabetes
  • have a history of disordered eating
  • have anxiety or depression

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What are the health benefits of fasting?

1. Making weight loss easier

Intermittent fasting can change the function of hormones, cells and genes, Sawyer says. "Changes in hormone levels makes stored body fat more accessible and initiates important cellular repair processes. When you fast, insulin levels drop, and human growth hormone (HGH) increases. Your cells also initiate important cellular repair processes and change which genes they express."

2. Lowering the risk of type 2 diabetes

Anything that reduces insulin resistance should help lower blood sugar levels and protect against type 2 diabetes, says Sawyer. "Interestingly, intermittent fasting has been shown to have major benefits for insulin resistance and can lead to an impressive reduction in blood sugar levels." (Via a study in the Translational Research journal).

3. Reducing stress and inflammation in the body

Sawyer adds that "Studies show intermittent fasting can reduce oxidative damage and inflammation in the body. This should have benefits against ageing and development of numerous diseases, according to a study linked with the University of Toronto."

4. Supporting brain function

Because fasting has been linked to reduced inflammation, it is also believed to improve conditions associated with inflammation - which include Parkinson’s and Alzheimer’s - as well as improving memory and brain processing.

A 2007 study in the Neurobiology of Disease journal found that intermittent fasting can protect against cognitive decline in Alzheimer's disease when testing on mice, while the findings of another study in the Journal of Neuroscience Research suggested fasting may prove beneficial in reducing the incidence of Parkinson's in humans.

5. Improving heart health

"Intermittent fasting can improve numerous risk factors for heart disease," Sawyer says. "This includes blood pressure, cholesterol levels, triglycerides, and inflammatory markers." 

Though she does offer a word of caution, saying "Much of this research has been done on animals and therefore needs to be studied more in-depth in humans."

If you want to find out the truth behind more about popular diets and weight loss trends, we've asked the experts to explain the 7 day detox plan and the 1,000 calorie meal diet too. We've also taken a look at whether it's safe to lose 2 stone in 8 weeks.

Suzie Sawyer
Suzie Sawyer

Suzie Sawyer is a nutritional expert with more than 20+ years of experience. Specialising in female health and food supplements, she is a member of the British Association for Applied Nutrition and has a diploma in nutrition from the Institute for Optimum Nutrition, London. Suzie has been featured in a range of title, including Women’s Health, woman&home, Women, Women’s Own, and Daily Mail. 

Grace Walsh
Features Writer

Grace Walsh is a health and wellbeing writer, working across the subjects of family, relationships, and LGBT topics, as well as sleep and mental health. A digital journalist with over six years  experience as a writer and editor for UK publications, Grace is currently Health Editor for womanandhome.com and has also worked with Cosmopolitan, Red, The i Paper, GoodtoKnow, and more. After graduating from the University of Warwick, she started her career writing about the complexities of sex and relationships, before combining personal hobbies with professional and writing about fitness. 

With contributions from